1. Field of the Invention
The invention relates to immersion-type zinc phosphating systems and particularly to methods for controlling iron content and sludge accumulation in the processing bath tanks of such systems.
2. Description of the Prior Art
In applying zinc phosphate coating to steel parts in immersion-type phosphating systems, the dissolved iron content of the phosphating bath solution must be controlled to prevent retardation of the deposition of zinc phosphate in the coating process, i.e. to maintain the efficiency of an effectively monitored bath.
One past, and still too prevalent, practice has been to frequently shut down the system and dump the phosphating bath which has reached an unacceptable dissolved iron concentration level, clean the processing tank to remove accumulated sludge, pour a new bath, and then reactivate the system. The obvious disadvantages of this method are excessive equipment down time, nonproductive labor expenditures, substantially increased chemical consumption, and variations in the quality and uniformity of the phosphate coating on work being processed. In addition, the dumping of contaminated baths severely taxes the digestive capacities of local sewage treatment facilities and provides for a massive injection of pollutants where sumps continue to be drained directly into watercourses.
A somewhat more economical, but less than optimum, method of control has been to precipitate dissolved iron out of the bath by such means as supplying chemical additives to the bath or periodic batch aeration. The iron precipitate settles and builds up as sludge on the sides and base of the processing tank and periodically the system is shut down to permit scraping and removal of the accumulated sludge from the tank. Frequently, the phosphating bath is temporarily pumped to a storage tank while cleaning of the processing tank is carried out with the bath being later pumped back into the processing tank and brought up to a specified operating level. While the latter procedures are certainly preferable to that first mentioned from the aspects of pollution control and reduction of zinc phosphate consumption, they nevertheless require periodic equipment shutdown, expenditures for nonproductive labor, expenditures for chemical additives to precipitate the iron, expenditures for storage tanks and pumping equipment if the bath is to be temporarily drained and reconstituted after tank cleaning, expenditures for batch aeration equipment if that method of precipitating the iron is to be employed in lieu of chemical precipitation, and possible expenditures for one or more standby systems to be utilized while a primary system is being batch aerated and desludged. Further, chemical additives to promote iron precipitation frequently adversely affect the quality of zinc phosphate coatings.
Iron precipitation combined with continuous decanting and replacement of the bath solution can be employed to extend the time interval between shutdowns for complete processing tank cleaning and desludging. Decanting procedures, however, have proved costly regarding chemical consumption and present a pollution problem which, although it is not as acute as batch dumping, recommends against this method of iron control.
Those familiar with the art will readily appreciate the difficulty in achieving uniformity of phosphate coating when the dissolved iron content of an operating bath is constantly varying, as it is when the aforementioned methods of iron content control are employed. Since the phosphating chemical requirements change relative to the change in iron content, i.e. a higher iron level must be compensated for or offset by a higher phosphate chemical level, extremely close monitoring is necessary to minimize bath fluctuation and maintaining a uniform operating level.
The present invention is seen to provide an apparatus for iron control and waste disposal which represents a marked improvement over the procedures mentioned above, without materially increasing the capital expenditures for equipment necessary to implement the method and at the same time improving the quality of zinc phosphate coatings. The merits of the invention's contribution to the art will become clearly evident to one who proceeds to a consideration of the detailed description of the invention which follows: